Add notification tester

[ghost]

Is there an existing request for this?

• I have searched the existing issues

Feature description

It's difficult to test UP notification issues with airgapped default MollySocket server. Having a test notifications button should help tremendously with troubleshooting all the pieces involved with notification delivery.

[p1gp1g]

It will never be possible to do that on an air gapped server, since molly can't communicate with the server.
Nevertheless, there is the connection ping command to check if everything is correctly setup. And starting with mollysocket v1.4.0, a ping is sent with new registration (to show a test notification)